1. Field of the Invention
This invention pertains to the field of producing carbon-free polycrystalline silicon by the pyrolysis of a gaseous silicon compound. More specifically, the present invention relates to an improved graphite chuck and to a method of protection for a graphite chuck used to support the heated starter filament which thermally decomposes the gaseous silicon compound such that silicon is deposited on the starter filament without carbon contamination.
2. Discussion of Related Art
Polycrystalline rods are primarily used as precursors for making single crystal rods for the semiconductor industry by either the float zone melting process or by the Czochralski crystal pulling technique. These single crystal rods are then processed to form silicon wafers from which silicon chips are made.
Generally, polycrystalline rods are made by the pyrolytic decomposition of a gaseous silicon compound, such as silane or a chlorosilane (e.g., trichlorosilane) on a rod-shaped, red-heated starter filament made preferably from a silicon seed rod or, alternatively, from a high-melting point metal having good electrical conductivity such as tungsten or tantalum. The principles of the design of present state-of-the-art reactors for the pyrolysis of silane and chlorosilanes are set forth in, for example U.S. Pat. Nos. 3,147,141, 4,147,814, and 4,150,168, which are incorporated herein by reference as if set out in full. It is generally more desirable to prepare the polycrystalline silicon rods by silane pyrolysis so as to avoid the complications caused by the formation of chloride by-products when pyrolyzing chlorosilanes.
The pyrolysis of silane to form silicon and hydrogen, or a chlorosilane which produces chloride-containing compounds such as HCl, SiHCL.sub.2 or the like as well as hydrogen, is performed in a reactor consisting of a series of heated filaments, generally silicon rods, surrounded by cooled surfaces. Typically, the filaments are heated by introducing an electrical current through the filament. The process is started with the silicon filament at ambient temperature.
The polycrystalline silicon is produced by heterogeneous decomposition of the silane or chlorosilane on the glowing hot silicon filament rod. The reaction deposits silicon on the surface of the rod and releases hydrogen gas if the silicon is formed by the decomposition of the silane, or hydrogen gas in conjunction with other chloride-containing by-product compounds if the source of the silicon is a chlorosilane.
One of the major objectives in the production of polycrystalline silicon is to produce a silicon rod which is as pure as possible. Even slight amounts of contaminants have a major impact on the efficacy of the silicon chips which are ultimately made from this precursor polycrystalline silicon. The prior art techniques for making polycrystalline silicon have had the problem of coping with various contaminants, including carbon. The present invention is specifically directed to reducing the amount of carbon which may be present in such a polycrystalline silicon rod.
While it has been recognized that carbon is undesirably present as a contaminant in the polysilicon rod, there has been no appreciation or recognition as to what the source of such carbon is or how it is contaminating the polysilicon rod. Indeed, without being able to determine the source for such contamination or the means by which such contamination takes place, a solution to this problem has long been awaited.